1. Field of Invention
This invention relates to a method of crosslinking or simultaneously crosslinking and-foaming of polymers using a high decomposition temperature azo-ester crosslinking agent.
2. Description of the Prior Art
Prior to the present invention, azo compounds generally were thought to be unable to abstract a hydrogen atom from a carbon atom and, hence, were unable to perform as a crosslinking agent for polymers. U.S. Pat. No. 3,152,107 was the first published exception to this general rule; it described the crosslinking effect of unsymmetrical aromatic azo compounds wherein one side of the azo group (--N.dbd.N--) is attached directly to the aromatic nucleus and the other side of the azo group is attached to an aliphatic radical with a strongly electrophilic group like a carbonamide group attached to the .alpha.-carbon atom thereof. Examples of such compounds are as follows: ##STR2## These azo compounds have not been found commercially acceptable hitherto, probably because of their relatively high price and their color which is presumably due to the presence of the chromophoric azo group attached to the aromatic ring; this color represents a severe disadvantage in many applications because decomposed residue of the crosslinking agent causes undesirable discoloration of the polymer.
U.S. Pat. No. 3,776,885 is a second exception to the abovementioned general rule; it describes aliphatic symmetrical and asymmetrical azo-ethers having the following structures: ##STR3## These azos are good crosslinking agents for polymers in the temperature range of 120.degree. to 230.degree. C. Nevertheless, when comparing these azo ethers (II) and (III) with the azo-ester (I) of the present invention, the azo ester has a higher decomposition temperature for analogously structured compounds, i.e., made from the same ketone. Other differences of the azo ester (I) of the present invention are their simpler and cheaper preparation, lower volatility, solid state (in many instances), and low or lack of toxicity. For example, the azo ether, 2,2'azo-bis(2-methoxy propane) is a toxic liquid while the corresponding azo ester, 2,2'azo-bis(2-acetoxy-propane) is a colorless power melting at 103.degree. C.; similarly the azo ether, 1,1'azo-bis(1-methoxycyclohexane) is an orange-yellow oil solidifying at about 0.degree. C. while the corresponding azo ester, 1,1'azo-bis(1-acetoxycyclohexane) is a colorless powder melting at 91.degree.-94.degree. C.
Organic peroxides and diperoxides are normally used as crosslinking agents for crosslinking polymers. Peroxides, however, cannot be used when the softening temperature of the polymer is above the decomposition temperature of the peroxide because the peroxide would decompose before being completely mixed with the polymer. On the other hand, the high decomposition temperature of the azo ester of this invention makes it possible to mix them with such polymers having a softening temperature higher than the decomposition temperature of the peroxides such as dicumyl peroxide, .alpha.,.alpha.'-bis(t-butylperoxy)-p(m)-diisopropylbenzene, 2,5-dimethyl-2,5-bis(t-butylperoxy)hexane, 2,5-dimethyl-2,5-bis(t-butylperoxy)hexyne(3), di-t-butylperoxide, etc. Azo esters can effect foaming during crosslinking in the absence of pressure; peroxides cannot. Crosslinking with the azo ester under pressure, however, gives compact unfoamed polymers in the same way and in the same order of magnitude as when peroxides are used.
The commercially available azo-compounds used technically as initiators are symmetrical azonitriles of the general structure: ##STR4## The most known representative of the azonitriles is 2,2'azo-bis-isobutyronitrile. The azonitriles have a decomposition temperature range of about 60.degree.-80.degree. C. below that of the abovementioned crosslinking peroxides and about 80.degree.-110.degree. C. below that of the azo-ethers (II) and (III) and the azo-ester (I) of the present invention. These azonitriles cannot effect crosslinking reactions in polymers (see Example 4, infra).